1. Field of the Invention
The present invention relates to a method for operating a hydraulic drive apparatus in which flows of pressurized oil discharged from a plurality of hydraulic drive sources each utilizing a hydraulic pump are merged in order to drive an actuator.
2. Description of the Related Art
A conventional hydraulic drive apparatus built into a large-sized injection molding machine is disclosed in, for example, Japanese Patent Publication (kokoku) No. 7(1995)-92061. In the hydraulic drive apparatus, in order to avoid an increase in cost stemming from employment of a single, large hydraulic drive source, a plurality of hydraulic drive sources are connected in parallel in order to merge the flows of pressurized oil discharged from the respective hydraulic drive sources, to thereby secure a large flow rate. The hydraulic drive sources generally have an identical circuit configuration, including a hydraulic pump.
In a speed control mode, a speed control signal determined on the basis of a speed command value is simultaneously supplied to the respective hydraulic drive sources, so that pressurized fluid is supplied from the hydraulic drive sources to various kinds of actuators such as hydraulic cylinders built into an injection apparatus and a clamping apparatus, and the speeds of the actuators are controlled by means of feedback control. In this case, the hydraulic drive sources discharge pressurized oil at the same flow rate. In a pressure control mode, a pressure control signal determined on the basis of a pressure command value is simultaneously supplied to the respective hydraulic drive sources, so that pressurized fluid is supplied from the hydraulic drive sources to the actuators, and the pressure of oil supplied to the actuators is controlled by means of feedback control.
However, the above-described conventional hydraulic drive apparatus has the following problem. In the pressure control mode, a pressure control signal is simultaneously supplied to the plurality of hydraulic drive sources of identical structure, and the pressure of discharged oil is controlled through feedback control in a state in which the flow rate is nearly zero. This control state causes pressure oscillation phenomenon (resonance phenomenon), resulting in impairment of operation stability. Therefore, pressure control cannot be performed with a high degree of accuracy and a high degree of reproducibility.
An object of the present invention is to provide a method for operating a hydraulic drive apparatus which secures stable operation and enables pressure control with a high degree of accuracy and a high degree of reproducibility, through reliable prevention of a pressure oscillation phenomenon which would otherwise occur when a plurality of hydraulic drive sources are connected in parallel.
Another object of the present invention is to provide a method for operating a hydraulic drive apparatus which can be performed through adjustment of existing components and can obviate necessity for additional parts and structures, thereby enabling the method to be performed easily and at low cost.
To achieve the above-described objects, the present invention provides a method for operating a hydraulic drive apparatus in which flows of oil discharged from a plurality of hydraulic drive sources each using a hydraulic pump are merged in order to drive an actuator, wherein at least in a pressure control mode, one of the hydraulic drive sources (main hydraulic drive source) is operated in a regular manner, and the other hydraulic drive source (auxiliary hydraulic drive source) is operated such that at a merge point where the oil flows merge, the pressure of oil from the auxiliary hydraulic drive source becomes lower than the pressure of oil from the main hydraulic drive by a predetermined amount. An ordinary pressure control signal is applied to the main hydraulic drive to perform an ordinary pressure control. However, through modification of the pressure control signal applied to the auxiliary hydraulic drive source and adjustment of a control valve, the pressure of oil from the auxiliary hydraulic drive source at the merge point becomes lower than the pressure of oil from the main hydraulic drive source at the merge point by a predetermined amount, so that pressure oscillation phenomenon (resonance phenomenon) is prevented.